1. Field of the Invention
The present invention relates to a fuel cell and a method for operating the same, the fuel cell comprising a fuel cell unit composed of an electrolyte interposed between an anode electrode and a cathode electrode, and separators for supporting the fuel cell unit interposed therebetween.
2. Description of the Related Art
For example, the solid polymer type fuel cell comprises a fuel cell unit including an anode electrode and a cathode electrode disposed opposingly on both sides of an ion exchange membrane composed of a polymer ion exchange membrane (cation exchange membrane) respectively, the fuel cell unit being interposed between separators. Usually, the solid polymer type fuel cell is used as a fuel cell stack obtained by stacking a predetermined number of the fuel cell units.
In such a fuel cell stack, a fuel gas such as a gas principally containing hydrogen (hereinafter referred to as xe2x80x9chydrogen-containing gasxe2x80x9d as well), which is supplied to the anode electrode, contains hydrogen which is ionized into ion on the catalyst electrode, and the ion is moved toward the cathode electrode via the electrolyte which is appropriately humidified. The-electron, which is generated during this process, is extracted for an external circuit, and the electron is utilized as DC electric energy. An oxygen-containing gas such as a gas principally containing oxygen or air (hereinafter referred to as xe2x80x9coxygen-containing gasxe2x80x9d as well) is supplied to the cathode electrode. Therefore, the hydrogen ion, the electron, and the oxygen are reacted with each other on the cathode electrode, and thus water is produced.
In the fuel cell described above, a variety of humidifying structures are adopted in order to avoid the degradation of power generation performance which would be otherwise caused when the ion exchange membrane is dried. For example, a method is known, in which the hydrogen-containing gas or the oxygen-containing gas is supplied to the fuel cell after being humidified to approximately give a dew point at an operating temperature of the fuel cell. However, in the case of the method described above, for example, the hydrogen-containing gas or the oxygen-containing gas is subjected to bubbling in hot water at about 80xc2x0 C. Therefore, it is necessary to provide additional equipment including a heater or the like. For this reason, the following inconvenience arises. That is, the entire equipment has a large size, and the equipment cost is expensive.
Considering such an inconvenience, in order to humidify the ion exchange membrane without using any additional humidifying apparatus, for example, a fuel cell is known, which is disclosed in U.S. Pat. No. 5,773,160. Water is produced by the reaction on the cathode electrode. The product water is increased in the flow direction of the oxygen-containing gas in the oxygen-containing gas flow passage for supplying the oxygen-containing gas to the cathode electrode. Therefore, the maximum water content region exists in the vicinity of the oxygen-containing gas outlet. On the other hand, the water content tends to decrease from the fuel gas inlet toward the fuel gas outlet in the ion exchange membrane on the side of the fuel gas flow passage for supplying the hydrogen-containing gas to the anode electrode. An inconvenience arises such that the ion exchange membrane is dried.
Accordingly, the fuel cell described above is constructed as follows. That is, the oxygen-containing gas flow passage, which is disposed in the vicinity of the oxygen-containing gas outlet, is arranged closely to the fuel gas flow passage which is disposed in the vicinity of the fuel gas inlet. Thus, the water, which is accumulated in the oxygen-containing gas flow passage, is subjected to back-diffusion toward the fuel gas flow passage in accordance with the concentration gradient of water to humidify the ion exchange membrane on the side of the fuel gas flow passage.
However, in the case of the conventional technique described above, the thickness of the ion exchange membrane is often set to be about 100 xcexcm. When such a large thickness is used, it is impossible for the water to appropriately cause the back-diffusion from the side of the oxygen-containing gas flow passage to the side of the fuel gas flow passage. For this reason, the following problem is pointed out. That is, the humidity retention is not achieved sufficiently for the ion exchange membrane on the side of the anode electrode, and it is impossible to maintain the stable power generation performance.
Further, no water is produced in the oxygen-containing gas flow passage upon the start of operation of the fuel cell. Therefore, a problem arises such that the ion exchange membrane is in a dried state, and the power generation performance is unstable.
A general object of the present invention is to provide a fuel cell which makes it possible to reliably supply, toward the side of a fuel gas flow passage, the water produced in an oxygen-containing gas flow passage and which makes it possible to perform non-humidifying operation with a simple structure.
A principal object of the present invention is to provide a method for operating a fuel cell, which makes it possible to perform the non-humidifying operation and which makes it possible to effectively shorten the start-up time.
The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present invention is shown by way of illustrative example.